In the aircraft industry, the use of lightweight fiber reinforced composite materials has been steadily increasing over the past few years. As the use of such materials expands, there will be an increased need for a cost effective method of producing high quality holes in such materials with dimensions within narrow tolerances. A number of problems arise in using known drilling and grinding techniques to produce holes in fiber reinforced composite materials. These problems include delamination and splintering of the material and fiber breakout. Such phenomena are unacceptable in most applications for a number of reasons, including weakening of the structure through which the hole extends around the hole area.
The problems discussed above were addressed in a prior patent application of the present applicants, now U.S. Pat. No. 4,720,218, granted Jan. 19, 1988. The patent discloses three embodiments of a combination drilling and dressing bit that was developed to produce holes in fiber reinforced composite materials, such as graphite fiber reinforced bismaleimide (BMI). More specifically, the combination bit was developed to drill holes in cargo heat duct tubes made from graphite reinforced BMI. The disclosure of the patent is incorporated herein by reference, and the combination bits described in the patent will only be briefly described herein.
Each of the three embodiments of the bit disclosed in the patent has a pointed tapered drill portion for producing a hole, a frustoconical grinder portion for enlarging the hole, and a cylindrical reamer portion for finishing the hole. In one embodiment, the drill portion and the grinder portion together form a substantially continuous taper. In another embodiment, the inner end of the drill portion is wider than the outer end of the frustoconical grinder portion to permit the grinder portion to gradually engage the sidewall of a hole being drilled. The drill portion in each of these two embodiments has four flat faces which meet along four edges. Each of the four edges may be sharp to form a cutting edge, as described in the patent. Alternatively, the drill portion may be wider between two opposite edges than between the other two edges so that these opposite edges do all of the actual cutting. This arrangement helps avoid the problem of precisely dimensioning the drill portion so that all four edges make contact and participate in the cutting.
The third embodiment disclosed in the patent has a flattened drill portion with two major faces and two minor faces forming a cross section in the shape of a parallelogram. The two opposite edges which define the widest part of the drill portion do the cutting. The positioning of these edges can be seen in FIG. 8 of the patent.
The combination bits disclosed in the patent and briefly described herein worked well for their intended purpose, i.e., for drilling holes in cargo heat duct tubes. However, problems were encountered with the use of these bits when it was attempted to scale down the bits to drill much smaller holes. The smaller holes were required in the development of composite material anti-icing ducts. In such ducts, holes with diameters of about 0.141 inch, for example, are required. Attempts to scale down the most preferred embodiment disclosed in the patent, the embodiment with the flattened drill portion, by making the drill portion longer were unsuccessful. The elongated tip was too thin and fragile. When used to drill small holes, the patented combination bits also presented the problem of excessive vibration which in turn caused excessive wear of the bits and poor quality holes.
U.S. Pat. No. 2,715,772, granted Aug. 23, 1955, to A. Fritz, discloses a dental burr for drilling root canals. The patent addresses problems encountered in connection with previous dental burrs, including clogging of longitudinal grooves, short useful life, and holes that do not fit the root pin after the burr is sharpened. The Fritz burr has an obtuse-angled pyramid point and a cutting portion with cutting edges that extend parallel to the burr axis and then taper toward the point. The cutting portion has a generally triangular cross section with concave sides. This arrangement allows the cutting portion to continue drilling holes sized to tapered root pins even after resharpening. The concave faces allow dentists to resharpen the burr themselves.
U.S. Pat. No. 456,258, granted July 21, 1891, to D. B. Hutton et al., discloses a drill for drilling square holes. The drill has a triangular shank with radial cutting edges formed by bevelling the end of the shank. The shank has rounded sides to prevent jarring experienced with flat-sided drills. U.S. Pat. No. 3,429,171, granted Feb. 25, 1969, to J. J. Feher, discloses a drill for piercing and extruding sheet metal. The drill has a tapering terminal end portion that is free of edges which would effect a cutting action in a manner such to remove material. The absence of cutting edges allows the drill to form a protrusion on the back side of the sheet metal to increase the extent of the hole and thereby provide a better grip for a fastener. The end portion is described as being conical or having a generally triangular or some other cross-sectional shape as long as it is free of edges that would effect cutting.
U.S. Pat. No. 4,507,028, granted Mar. 26, 1985, to M. Matsushita, discloses a cutting tool with a front drill part and a rear reamer part. Both parts include a pair of twist drill flutes. In one embodiment, the flutes are continuous between the parts, and the parts apparently have the same diameter. In another embodiment, the parts are separated by a neck, and the reamer part has a chamfer to ream within the finishing allowance left by the drill part and a generally cylindrical portion for finishing the hole. The diameter of the cylindrical portion is apparently essentially the same as the diameter of the drill part. A third embodiment is in the form of a step drill with two drill parts and two reamer parts, all with different diameters.
U.S. Pat. No. 4,091,525, granted May 30, 1978, to C. Karasiewicz et al., discloses a reamer for fine reaming of ports. The reamer has a blunt tapered working portion with cutting blades and a tapered rear guiding portion with longitudinally extending smoothing inserts. The working portion has longitudinal flutes and five cutting blades. The edges of the blades and the inserts in the guiding portion lie on the surface of an imaginary cone.
Drills for producing holes in composite materials are disclosed in U.S. Pat. No. 4,093,395, granted June 6, 1978, to W. K. Luebbert et al; U.S. Pat. No. 4,285,620, granted Aug. 25, 1981, to W. K. Luebbert et al.; U.S. Pat. No. 4,440,532, granted Apr. 3, 1984, to S. D'Apuzzo; and U.S. Pat. No. 4,725,171, granted Feb. 16, 1988, to R. P. DeTorre. The D'Apuzzo drill bit has three semicircular longitudinal grooves between three cutting elements. The tip of the bit is blunt and has three nearly radial cutting edges. An additional three curved cutting edges are defined by the grooves. The radial cutting edges perforate the material and the curved cutting edges ream the hole to full size. The earlier Luebbert et al. drill has a flattened cutting tip with two major faces, two minor faces, and two cutting edges. The later Luebbert et al. drill is in the form of a two-fluted twist drill or straight fluted drill with two cutting edges on the end to scribe a circle and two longitudinal or spiral cutting edges to cut away the core of the hole. The DeTorre drill has a tapering planar eliptical end surface and a radial flute that decreases in the axial direction.
Cutting tools with generally cone-shaped tips and single cutting edges formed by a slot are disclosed in U.S. Pat. Nos. 3,076,356, granted Feb. 5, 1963, to E. Simich; and U.S. Pat. No. 3,836,278, granted Sept. 17, 1974, to A. G. P. McInnes. U.S. Pat. No. 4,560,309, granted Dec. 24, 1985, to C. R. Hornsby, discloses a tapered finishing reamer with two opposite longitudinally-extending teeth. U.S. Pat. No. 3,688,386, granted Sept. 5, 1972, to A. J. Pereira, discloses a method of forming holes in contact lenses which employs a five-sided reamer.